Length artifact and method of measurement

ABSTRACT

A length artifact has a first side wall, a second side wall, and a base portion, the first side wall and the second side wall separated by an air gap at least four millimeters wide, the base portion being attached to a bottom of the first side wall and the second side wall, the base portion further having a first platform region that includes a first nest and a second platform region that includes a second nest, the first nest and the second nest configured to accept a spherical surface of a retroreflector target.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims the benefit ofU.S. Non-Provisional patent application Ser. No. 14/662,450, filed Mar.19, 2015, and U.S. Provisional Patent Application No. 62/010,043, filedJun. 10, 2014, the contents of both of which are incorporated byreference herein.

BACKGROUND OF THE INVENTION

The present disclosure relates to a length artifact to determine themeasurement errors associated with a three-dimensional (3D) measurementinstrument and with methods of measurement using the length artifact.

Portable metrology instruments, such as portable laser trackers andarticulated arm coordinate measuring machines (AACMMs), find widespreaduse in the manufacturing or production of parts where there is a need torapidly and accurately verify the dimensions of the parts during variousstages of the manufacturing or production (e.g., machining). Portablemetrology instruments represent an improvement over known stationary orfixed, cost-intensive and relatively difficult to use measurementinstallations, particularly in the amount of time it takes to performdimensional measurements of relatively complex parts.

A laser tracker is in a class of instruments that measure coordinates ofa point by sending a beam of light to the point. The beam of light mayimpinge on a retroreflector target in contact with the point. The lasertracker determines the coordinates of the point by measuring thedistance and the two angles to the target. The distance is measured witha distance-measuring device such as an absolute distance meter (ADM) oran interferometer. The angles are measured with an angle-measuringdevice such as an angular encoder. A gimbaled beam-steering mechanismwithin the instrument directs the laser beam to the point of interest. Arelated instrument is a total station (tachymeter) that measures toeither a retroreflector or a point on a diffusely scattering surface.Laser trackers, which typically have accuracies on the order of athousandth of an inch and as good as one or two micrometers undercertain circumstances, are usually much more accurate than totalstations. The broad definition of laser tracker, which includes totalstations, is used through this application.

Ordinarily the laser tracker sends a laser beam to a retroreflectortarget. A common type of retroreflector target is the sphericallymounted retroreflector (SMR), which comprises a cube-cornerretroreflector embedded within a metal sphere. The cube-cornerretroreflector comprises three mutually perpendicular mirrors. Thevertex, which is the common point of intersection of the three mirrors,is located at the center of the sphere. Because of this placement of thecube corner within the sphere, the perpendicular distance from thevertex to any surface on which the SMR rests remains constant, even asthe SMR is rotated. Consequently, the laser tracker can measure the 3Dcoordinates of a surface by following the position of an SMR as it ismoved over the surface. Stating this another way, the laser trackerneeds to measure only three degrees of freedom (one radial distance andtwo angles) to fully characterize the 3D coordinates of a surface.

Periodically it is desirable to evaluate the performance of a 3Dmeasuring instrument. One way to do this is to measure a lengthartifact, also known as a length standard or a reference length, and tocompare a length of the artifact as obtained from a calibration to thelength as obtained from a measurement with the 3D measuring instrument.The difference between the measured and calibrated length values is theerror, which is an indicator of performance of the 3D measuringinstrument. Today, there is a need for a length artifact that is stable,easily calibrated, and easily measured with a 3D measuring instrumentsuch as a laser tracker, articulated arm CMM, or similar device.

Accordingly, while existing length artifacts are suitable for theirintended purposes, the need for improvement remains, particularly inproviding an improved length artifact and method of measuring with a 3Dmeasuring device.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with an embodiment, a length artifact is provided, thelength artifact having an artifact frame of reference, the lengthartifact including a first side wall and a second side wall, the firstside wall and the second side wall separated by an air gap, the air gapbeing at least four millimeters wide, the first side wall having a firstbottom surface, a first left surface, and a first right surface, thesecond side wall having a second bottom surface, a second left surface,and a second right surface; and a base portion in contact with the firstbottom surface and the second bottom surface, the base portion having afirst platform region and a second platform region, the first platformregion extending beyond the first left surface and the second leftsurface, the second platform region extending beyond the first rightsurface and the second right surface, the first platform regionincluding a first nest, the first nest configured to accept a sphericalsurface of a retroreflector target, the spherical surface having asphere center, the second platform region including a second nest, thesecond nest configured to accept the spherical surface of theretroreflector target, the length artifact further including a firstnest center and a second nest center, the first nest center being firstthree-dimensional (3D) coordinates in the artifact frame of reference ofthe sphere center when the retroreflector target is placed in the firstnest, the second nest center being second 3D coordinates in the artifactframe of reference of the sphere center when the retroreflector targetis placed in the second nest, the length artifact further having aneutral surface in the artifact frame of reference, the neutral surfacebeing a surface above which the length artifact is in tension and belowwhich the length artifact is in compression in accordance with anembodiment.

In accordance with another embodiment, a method of calibrating a lengthartifact is provided, the method including providing a retroreflectortarget configured to retroreflect a beam of light, the retroreflectortarget having a spherical portion, the spherical portion having a spherecenter; providing the length artifact, the length artifact having afirst side wall, a second side wall, and a base portion, the first sidewall and the second side wall separated by an air gap, a bottom portionof the first side wall and a bottom portion of the second side wall incontact with the base portion, the base portion including a first nestand a second nest, the first nest configured to accept the sphericalportion when the sphere center is at a first nest center, the secondnest configured to accept the spherical portion when the sphere centeris at a second nest center; providing a laser tracker, the laser trackerconfigured to project a beam of light to the retroreflector target, totrack the retroreflector target, and to determine three-dimensional (3D)coordinates of the retroreflector target, the 3D coordinates based atleast in part on a distance from the laser tracker to the retroreflectortarget, the distance determined by a distance meter within the lasertracker, the distance based at least in part on a speed of light in air;aligning the beam of light with the first nest center and the secondnest center; placing the retroreflector target in the first nest;sending the beam of light to the retroreflector target; measuring withthe laser tracker a first distance to the first nest center; placing theretroreflector target in the second nest; sending the beam of lightthrough the air gap between the first side wall and the second side wallto the retroreflector target; measuring with the laser tracker a seconddistance to the second nest center; and determining a first artifactlength based at least in part on the first distance and the seconddistance

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter, which is regarded as the invention, is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1A is a perspective view of a laser tracker in accordance with anembodiment of the invention;

FIG. 1B is a block diagram showing elements of tracker electronics inaccordance with an embodiment of the invention;

FIG. 2 is a perspective view of a portable articulated arm coordinatemeasuring machine (AACMM) in accordance with an embodiment of theinvention;

FIG. 3 is a perspective view of a prior art method for establishing areference length;

FIGS. 4A-G are views of a length artifact: perspective, top, front, leftside, right side, section, and nest-SMR detail, respectively, inaccordance with an embodiment of the invention;

FIGS. 5A-C are views of a mount according to embodiments;

FIGS. 6A-C are schematic illustrations of the effect of retroreflectortarget placement on a length artifact;

FIGS. 7A and 7B are perspective and side views, respectively, of anartifact that includes a temperature sensor; and

FIG. 8 is a flowchart of a method of determining a length of a lengthartifact according to an embodiment.

The detailed description explains embodiments of the invention, togetherwith advantages and features, by way of example with reference to thedrawings.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention provides a length artifact havingfeatures that provide convenience, stability, and accuracy in theevaluation of the performance of 3D measuring devices such as lasertrackers and articulated arm CMMs.

An exemplary laser tracker system 5 illustrated in FIG. 1A includes alaser tracker 10, a retroreflector target 26, an optional auxiliary unitprocessor 50, and an optional auxiliary computer 60. An exemplarygimbaled beam-steering mechanism 12 of laser tracker 10 comprises azenith carriage 14 mounted on an azimuth base 16 and rotated about anazimuth axis 20. A payload 15 is mounted on the zenith carriage 14 androtated about a zenith axis 18. Zenith axis 18 and azimuth axis 20intersect orthogonally, internally to tracker 10, at gimbal point 22,which is typically the origin for distance measurements. A laser beam 46virtually passes through the gimbal point 22 and is pointed orthogonalto zenith axis 18. In other words, laser beam 46 lies in a surface(plane) approximately perpendicular to the zenith axis 18 and thatpasses through the azimuth axis 20. Outgoing laser beam 46 is pointed inthe desired direction by rotation of payload 15 about zenith axis 18 andby rotation of zenith carriage 14 about azimuth axis 20. A zenithangular encoder, internal to the tracker, is attached to a zenithmechanical axis aligned to the zenith axis 18. An azimuth angularencoder, internal to the tracker, is attached to an azimuth mechanicalaxis aligned to the azimuth axis 20. The zenith and azimuth angularencoders measure the zenith and azimuth angles of rotation to relativelyhigh accuracy. Outgoing laser beam 46 travels to the retroreflectortarget 26, which might be, for example, a spherically mountedretroreflector (SMR) as described above. By measuring the radialdistance between gimbal point 22 and retroreflector 26, the rotationangle about the zenith axis 18, and the rotation angle about the azimuthaxis 20, the position of retroreflector 26 is found within the sphericalcoordinate system of the tracker.

FIG. 1B is a block diagram depicting a dimensional measurementelectronics processing system 1500 that includes a laser trackerelectronics processing system 1510, processing system of a temperaturesensor peripheral 1582, computer 1590, and other networked components1600, represented here as a cloud. Exemplary laser tracker electronicsprocessing system 1510 includes a master processor 1520, payloadfunctions electronics 1530, azimuth encoder electronics 1540, zenithencoder electronics 1550, display and user interface (UI) electronics1560, removable storage hardware 1565, radio frequency identification(RFID) electronics 1570, and an antenna 1572. The payload functionselectronics 1530 includes a number of subfunctions including the six-DOFelectronics 1531, the camera electronics 1532, the ADM electronics 1533,the position detector (PSD) electronics 1534, and the level electronics1535.

FIG. 2 shows an exemplary AACMM 200 that may comprise a six or sevenaxis articulated measurement device having a probe end 201 that includesa measurement probe housing 202 coupled to an arm portion 204 of theAACMM 200 at one end. The arm portion 204 comprises a first arm segment206 coupled to a second arm segment 208 by a rotational connectionhaving a first grouping of bearing cartridges 210 (e.g., two bearingcartridges). A second grouping of bearing cartridges 212 (e.g., twobearing cartridges) couples the second arm segment 208 to themeasurement probe housing 202. A third grouping of bearing cartridges214 (e.g., three bearing cartridges) couples the first arm segment 206to a base 216 located at the other end of the arm portion 204 of theAACMM 200. Each grouping of bearing cartridges 210, 212, 214 providesfor multiple axes of articulated movement. Also, the probe end 201 mayinclude a measurement probe housing 202 that comprises the shaft of theseventh axis portion of the AACMM 200 (e.g., a cartridge containing anencoder system that determines movement of the measurement device, forexample a contact probe tip 218, in the seventh axis of the AACMM 100).In this embodiment, the probe end 201 may rotate about an axis extendingthrough the center of measurement probe housing 202. In use the base 216is typically affixed to a work surface.

Each bearing cartridge within each bearing cartridge grouping 210, 212,214 typically contains an encoder system (e.g., an optical angularencoder system). The encoder system (i.e., transducer) provides anindication of the position of the respective arm segments 206, 208 andcorresponding bearing cartridge groupings 210, 212, 214 that alltogether provide an indication of the position of the probe 218 withrespect to the base 216 (and, thus, the position of the object beingmeasured by the AACMM 100 in a certain frame of reference—for example alocal or global frame of reference).

The probe tip 218 is detachably mounted to the measurement probe housing202, which is connected to bearing cartridge grouping 212. In exemplaryembodiments, the probe housing 202 houses a removable probe tip 218. Inother embodiments, the measurement is performed, for example, by anon-contacting device such as a laser line probe (LLP). In anembodiment, the handle 226 is replaced with the LLP using thequick-connect interface. Other types of accessory devices may replacethe removable handle 226 to provide additional functionality.

In accordance with an embodiment, the base 216 of the portable AACMM 200contains or houses an electronic data processing system that includes abase processing system that processes the data from the various encodersystems within the AACMM 200 as well as data representing other armparameters to support 3D positional calculations. An electronic dataprocessing system in the base 216 may communicate with the encodersystems, sensors, and other peripheral hardware located away from thebase 216 (e.g., a LLP that can be mounted to or within the removablehandle 226 on the AACMM 200). The electronics that support theseperipheral hardware devices or features may be located in each of thebearing cartridge groupings 210, 212, 214 located within the portableAACMM 200.

FIG. 3 shows a prior art method for establishing a reference length Lbetween a retroreflector target in a first position 320A and a secondposition 320B. The laser tracker 310 includes a calibrated distancemeter such as an interferometer or ADM. For this measurement of a lengthL between two points, an appropriately calibrated laser tracker may havemetrological traceability whereby the result of the measured length canbe related to stated references, usually national or internationalstandards, through an unbroken chain of comparisons all having stateduncertainties. The error in the measured length L of the referenceartifact is minimized by aligning the beam of light 315A from the lasertracker 310 to the positions 320A, 320B of the retroreflector target.The reason for the reduced error is that the length L is determinedbased only on the readings of the distance meter and not only the anglereadings of the tracker 310, which are less accurate.

The retroreflector target may be an SMR similar to element 26 of FIG.1A. An SMR, which has a spherically shaped outer portion surface, may berepeatably placed in a suitable nest. An example of a nest that mayprovide repeatable positioning for the spherically shaped outer portionof an SMR is a nest that includes three small spheres spaced apart by120 degrees. The larger spherical surface of the SMR makes contact atthe same points on the three spheres regardless of how the sphere isrotated. The vertex of an open-air cube corner retroreflector isadjusted to coincide with the center of the sphere so that the vertexalso remains at a fixed location as the SMR is rotated in the nest. Bythis means, the center of the SMR may be repeatably measured atpositions 320A, 320B, thereby enabling repeatable determination of thelength L. The nests at positions 320A, 320B are placed on instrumentstands 335, 340, respectively. The positions 320A, 320B correspond tothe centers of the retroreflector when placed on the nests of the stands335, 340, respectively. The beam of light 315A is sent from the tracker310 on the instrument stand 330 to the retroreflector at position 320A.The beam of light returns to the tracker 310 along the same beam path.The retroreflector is moved to the second position 320B and the beam oflight 315B is sent to and returns from the retroreflector in the newposition.

After a reference length L has been determined in calibration laboratoryaccording to the arrangement of FIG. 3, other measuring instruments maybe tested using the test setup 300 to evaluate their performance(determine their measurement errors). 3D instruments under test may beplaced in a variety of positions relative to the positions 320A, 320B.For example, a laser tracker may be placed to the side of the instrumentstands so as to use its angular measuring capability in addition to itsdistance measuring capability. In this way, the performance of thetracker may be determined for a variety of measurement conditions.

The method of establishing a reference length L shown in FIG. 3 issuited to a relatively permanent installation. It is less well suited toa quick check of the performance of a 3D instrument, especially if thereis a need to test instrument performance in a variety of locations. Forthis purpose, it is preferable to provide a physical artifact having areference length rather than the arrangement of FIG. 3. It is desirablethat such a physical artifact have metrological traceability asdescribed above.

A common (prior art) physical artifact having a reference length is aballbar. A ballbar includes a sphere attached to and centered on eachend of cylindrical shaft. The distance between the centers of the twospheres is measured in a calibration laboratory to provide a traceablereference length. The performance of a 3D instrument is established bymeasuring the length between the two spheres. The reference length issubtracted from the length measured by the 3D instrument to obtain anerror. This error is compared to a maximum permissible error (MPE) givenin a data sheet provided by the manufacturer of the 3D instrument. Bycomparing the observed error to the specified MPE value, the instrumentis found to be within the manufacturer's specifications or outside themanufacturer's specifications.

The 3D instrument under test determines the distance between the ballbarsphere centers by measuring the 3D coordinates of the sphere surface ata number of positions on the surface. This data is used in amathematical optimization procedure to obtain a best-fit sphere having asphere center. The 3D coordinates of the centers of the two best-fitspheres at the positions 320A, 320B are used to find the ballbar length.

A practical difficulty in using a ballbar to determine the performanceof a laser tracker is that it is difficult to obtain an acceptablesample of 3D coordinates for the sphere farthest from the tracker. Inother words, suppose the spheres of a ball bar were located in thepositions 320A and 320B of FIG. 3 (with the ballbar cylindrical shaftconnecting the two spheres). If the laser tracker were aligned to theballbar spheres as in FIG. 3, it would be difficult or impossible forthe tracker 310 to measure 3D coordinates of the near and far sides ofthe farthest ballbar sphere (at the position 320B). The portion of thefarthest sphere nearest the tracker is obscured by the cylindrical shaftof the ballbar. The portion of the farthest sphere that is farthest fromthe tracker cannot be contacted with the tracker SMR without pointingthe retroreflector away from the beam 315B, thereby breaking the beam.Therefore, for the farthest sphere in the ballbar, points can only becollected for the sphere central region, decreasing the ability toaccurately fit the collected data to a sphere. This has the undesirableconsequence of increasing the uncertainty in the tracker measurement.

Another practical difficulty in using a ballbar to measure devices suchas laser trackers or AACMMs is that the ballbar needs to be mounted attwo positions, ordinarily underneath the two spheres. As in the case ofthe arrangement shown in FIG. 3, ballbars are more suited to permanent,rather than flexible or impromptu, measurements of instrumentperformance. Another disadvantage of using a ballbar is that multiplepoints must be measured and a best-fit procedure carried out.

Another prior art method for establishing a reference length to verifythe performance of a laser tracker or AACMM is described in U.S. Pat.No. 8,051,575 ('575) to Bridges, et al., the contents of which areherein incorporated by reference. This patent describes a method inwhich a mount applied at two different positions along a length artifactmay place the neutral surface of the mounted artifact to coincide withthe SMR centers, thereby minimizing measurement error. An example ofsuch a mounted artifact is shown in FIG. 3 of the '575 patent. This typeof artifact and mounting arrangement is well suited to relatively largeartifacts but is unnecessarily complex for measurements in which thelength artifact can be mounted on a single structure or stand.

The mounted length artifact 490 described by FIGS. 4A-G overcome thelimitations of the prior art methods described hereinabove. The mountedlength artifact 490 includes a length artifact 400, which is optionallyattached to a mount 450. The length artifact 400 includes a base portion405 that extends the length of the artifact. Attached to the baseportion 405 are a first sidewall 425 and a second sidewall 430. Betweenthe first sidewall and the second sidewall is an air gap 435, the airgap being wide enough to permit passage of a beam of light from a lasertracker from one end of the artifact to the other end. In practice, thismeans that the air gap needs to be at least four millimeters wide. Eachof the first sidewall and the second sidewall has a left surface as seenin FIG. 4D, a right surface as seen in FIG. 4E, and a bottom surface,which is in contact with the base portion 405. In the embodimentillustrated in FIG. 4, the base portion, first sidewall, and secondsidewall are formed or machined from a single piece of material, butthese three components could be fabricated separately.

The base portion has a first platform region and a second platformregion, the first platform region extending beyond the left surfaces ofthe sidewalls and the second platform extending beyond the rightsurfaces of the sidewalls. The first platform includes a first nest 410Aand the second platform includes a second nest 410B. In FIG. 4, an SMR470 is shown placed on the second nest 410B. Each nest 410A, 410Bincludes contact points 412 that make contact with an outer sphericalportion 472 of the SMR 470 to permit repeatable positioning of the SMR,usually to within one or two micrometers. A magnet 414 may be includedin each of the nests 410A, 410B to securely hold the SMR in place. Acavity 474 in the SMR is sized to hold a cube-corner retroreflector (notshown), the cube corner having three perpendicular reflectors that forma cube-corner vertex. The SMR vertex coincides with the sphere center478 of the spherical portion 472. The SMR 470 may include a collar 476to simplify handling of the SMR by an operator. Recesses 415 may beplaced to either side of the nests 410A, 410B to accommodate the collar476.

The base portion 405, first sidewall 425, and second sidewall 430 may beformed from a single piece of material. In an embodiment, the materialis a metal having a low coefficient of thermal expansion (CTE) tominimize changes in artifact length over temperature. Examples of lowCTE material are Invar (CTE from 0.5 to 2 μm/m/T) and Super Invar (CTEfrom 0.3 to 1 μm/m/° C.). In another embodiment, the material is anon-metal having a low CTE. An example of such a material is acarbon-fiber composite, which may have a CTE less than 1 μm/m/° C. Otherlow CTE materials include certain ceramics and glasses. In otherembodiments, the artifact 400 is made of material that does not have alow CTE. Examples of such materials include steel (CTE approximately11.5 μm/m/° C.) and aluminum (CTE approximately 23 μm/m/° C.).

In an embodiment, the height and thickness of the first sidewall 425 andsecond sidewall 430 are selected relative to the height and width of thebase portion 405 so as to make the neutral plane (surface) 440 of theartifact 400 coincide with the center 478 of the SMR 475 when placed inthe nests 410A, 410B. This coincidence is shown in FIGS. 4C-E. A lengthartifact 400 has an artifact frame of reference 495 that is tied to theartifact. Suppose that the gravity vector in FIG. 6 is along the ydirection as shown in FIG. 6. Then the neutral surface is found bytaking the first moment of y over the mass of the structure. The lengthartifact 400 of FIG. 6 is made of homogeneous material and has most ofthe weight concentrated in the central portion that includes the firstand second sidewalls 425, 430. In this case, the neutral axis to a goodapproximation passes through the centroid of a cross section of thelength artifact 400 in a part of the artifact that includes the sidewalls. The neutral plane 440 passes through the neutral axis and lies inthe x-z plane.

In FIG. 4, the mount 450 attaches to the center of the artifact 400. Inone embodiment, the mount 450 screws onto an instrument stand. In anembodiment shown in FIGS. 5A, 5B, the mount 450 attaches to the lengthartifact 400 with four screws 455. In another embodiment, a mount 450attaches to the length artifact 400 with two screws 465 as shown in FIG.5C. Because the screws in either FIG. 5B or FIG. 5C are relatively closetogether, the support provided by the mount 450 can be modeled as asingle point 605 as shown in FIG. 6.

The distributed forces of gravity 620 of FIG. 6B cause the lengthartifact 400 to bend along a slight arc. Hence the neutral plane isactually a neutral surface. The portion of the length artifact above theneutral surface is in tension and the portion of the length artifactbelow the neutral surface is in compression. The reference length L ofthe length artifact 400 is determined along the x direction shown inFIG. 6. The gravity-induced angles of bending of the test artifact aresmall relative to the x direction and hence, for the case of the SMR 470placed on the neutral surface in the nests 410A, 410B, the error in themeasured length is just proportional to a cosine of the angle, which isa negligible error. In other words, in FIG. 6, the measured distance forthe SMR 470 placed on the nests 410A, 410B is L₁, which is very nearlyequal to the true length L. On the other hand, if the SMR 470 wereplaced on the upper surface of a length artifact as for the SMR 670 inFIG. 6C, the angle of tilt of the SMR 670 would cause the distancebetween the SMR center at the two nests to be increased to a distanceL₂. Even though the angle of tilt is not large, the center of the SMRalong the x direction shifts by a distance equal to the sine of theangle times the distance of the SMR above the neutral plane, which canbe a relatively large error. In other words, for a small angle of tiltθ, cos(θ)˜1−θ²/2, which is nearly equal to 1, while for the same smallangle, sin(θ)˜θ, which in the situation described hereinabove is likelyto be significantly larger than zero. By adjusting the neutral surfaceto coincide with the SMR centers, the error in the measured length ismade negligible. By placing the SMR center off the neutral axis, themeasured length may be significantly affected by gravity-inducedbending.

The error caused by bending is particularly important when a relativelylong artifact is supported by a single structure or stand. In this case,bending may be significant so that any changes in the test conditionscan cause a relatively large change in the measured length. For example,an AACMM may be tested with a length artifact 400. For example, thenests may be configured to accept spherical surfaces having a diameterof ⅞ inch. The AACMM may be fitted with a ball probe having a diameterof ⅞ inch. By placing the AACMM ball probe in each nest and measuringthe corresponding 3D coordinates in each case, the distance between thenest centers may be calculated. However, different operators may applydifferent amounts of force to the ball probe when putting it into thenest. An operator applying a greater amount of force will cause agreater amount of bending than would an operator applying a smalleramount of force. Hence, to obtain repeatable measurements using a lengthartifact 400, it is important that the neutral surface coincide as wellas possible to the nest (ball probe) centers. Similarly, different SMRs,even of a given diameter, may have different weights depending onconstruction. These weight differences can changes in the amount ofbending, resulting in different measured lengths if the SMR is measuredwith the SMR center off the neutral surface.

In some cases, it is desirable to use the same artifact on differenttypes of mounts. For example, in one instance, it may be desirable tomeasure the artifact having a mount shown in FIG. 5C. In anotherinstance, it may be desirable to remove the mount 450 and clamp thelength artifact 400 to a granite table. For SMRs (or ball probes) placedin nests so as to align the sphere centers with the neutral surface, thesame measured length L should be obtained in each case.

In an embodiment, a temperature sensor 780 is attached to a lengthartifact 700. The temperature sensor, which might be a thermistor, forexample, obtains an electrical signal that it sends to electronics 782that uses the received signal to determine a temperature. The electricalsignals may be sent to the electronics by over wires or wirelessly. Inan initial step, the reference length of the artifact is determinedusing a laser tracker having a calibrated distance meter. The referencetemperature T_(REF) of the artifact is recorded at the time of thismeasurement. Later the length of the artifact is measured with a 3Dinstrument to evaluate the accuracy of the instrument with the artifacthaving a test temperature T_(TEST). The length of the artifact iscorrected by an amount L*(T_(EST)−T_(REF))*CTE to account for thedifference in temperatures.

In a method 800 described in FIG. 8, a laser tracker is used todetermine a length of an artifact. A step 805 is providing a sphericalretroreflector target, which might be an SMR, for example. Theretroreflector includes a spherical portion that has a sphere center. Astep 810 is providing a length artifact. The length artifact has a firstside wall, a second side wall, and a base portion. The base portionincludes a first nest and a second nest configured to accept thespherical retroreflector, with the retroreflector center at the firstnest and the second being the first nest center and the second nestcenter, respectively. A step 815 is providing a laser tracker configuredto project light to the retroreflector target, track the retroreflectortarget, and measure 3D coordinates of the center of the retroreflectortarget.

A step 820 is aligning the laser tracker to the length artifact so thatthe beam of light from the tracker travels in a straight line along apath connecting the first nest center and the second nest center. A step825 is placing the retroreflector target in the first nest, which istaken here to be the nest closer to the tracker. It should be understoodthat the steps in the method 800 do not have to be carried out in theorder given, which means that order of measuring the near and far nestsmay be reversed. A step 830 is measuring with the laser tracker a firstdistance to the first nest center. A step 835 is placing theretroreflector target in the second nest. A step 840 is sending the beamof light through the air gap between the first side wall and the secondside wall to the retroreflector target. An example of a way of placingthe retroreflector target in the second nest is now given. Whiletracking the retroreflector with the beam of light, the retroreflectoris moved from the first nest over the top of the length artifact andthen placed into the second nest. The beam of light passes through theair gap to the retroreflector. A step 840 is measuring with the lasertracker a second distance to the second nest center. A step 845 isdetermining a first artifact length based at least in part on the firstdistance and the second distance.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the Figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims. Moreover, the useof the terms first, second, etc. do not denote any order or importance,but rather the terms first, second, etc. are used to distinguish oneelement from another. Furthermore, the use of the terms a, an, etc. donot denote a limitation of quantity, but rather denote the presence ofat least one of the referenced item.

What is claimed is:
 1. A length artifact having an artifact frame ofreference, the length artifact comprising: a first side wall and asecond side wall separated by an air gap; and a base portion in contactwith the first side wall and the second side wall, the base portionhaving a first platform region and a second platform region, the firstplatform region including a first nest sized and shaped to receive aspherical surface of a retroreflector target, the spherical surfacehaving a sphere center, the second platform region including a secondnest sized and shaped to receive the spherical surface of theretroreflector target, the length artifact further including a firstnest center and a second nest center, the first nest center being firstthree-dimensional (3D) coordinates in the artifact frame of reference ofthe sphere center when the retroreflector target is placed in the firstnest, the second nest center being second 3D coordinates in the artifactframe of reference of the sphere center when the retroreflector targetis placed in the second nest, the length artifact further having aneutral surface in the artifact frame of reference, the neutral surfacebeing a surface above which the length artifact is in tension and belowwhich the length artifact is in compression.
 2. The length artifact ofclaim 1, wherein the first nest center and the second nest center lie onthe neutral surface.
 3. The length artifact of claim 1, wherein the baseportion, the first side wall, and the second side wall are formed of afirst material.
 4. The length artifact of claim 3, wherein the firstmaterial is selected from the group consisting of Invar and Super Invar.5. The length artifact of claim 1, wherein the first nest includes afirst plurality of contact points and a first magnet and the second nestincludes a second plurality of contact points and a second magnet, thefirst plurality of contact points configured to make contact with thespherical surface when the retroreflector target is placed in the firstnest, the second plurality of contact points configured to make contactwith the spherical surface when the retroreflector target is placed inthe second nest, the first magnet configured to pull the retroreflectortarget toward the first platform region when the retroreflector targetis placed in the first nest, the second magnet configured to pull theretroreflector target toward the second platform region when theretroreflector target is placed in the second nest.
 6. The lengthartifact of claim 1, further including a mount configured for attachmentto the base portion.
 7. The length artifact of claim 6, wherein themount is attached to base portion with at least one screw.
 8. The lengthartifact of claim 6, wherein the mount is configured to screw onto aninstrument stand.
 9. The length artifact of claim 1, further including atemperature sensor in contact with a surface of the length artifact, thetemperature sensor configured to transmit an electrical signalindicative of a temperature of the length artifact.
 10. A method ofcalibrating a length artifact, the method comprising: providing aretroreflector target configured to retroreflect a beam of light, theretroreflector target having a spherical portion that includes a spherecenter; providing the length artifact having a first side wall, a secondside wall, and a base portion, the first side wall and the second sidewall separated by an air gap, the first side wall and the second sidewall in contact with the base portion, the base portion including afirst nest and a second nest, the first nest sized and shaped to receivethe spherical portion when the sphere center is at a first nest center,the second nest sized and shaped to receive the spherical portion whenthe sphere center is at a second nest center; providing a laser trackerconfigured to project a beam of light to the retroreflector target, andto determine three-dimensional (3D) coordinates of the retroreflectortarget, the 3D coordinates based at least in part on a distance from thelaser tracker to the retroreflector target; aligning the laser trackerto the length artifact so that the beam of light travels in a straightline along a path connecting the first nest center and the second nestcenter; placing the retroreflector target in the first nest; sending thebeam of light to the retroreflector target; measuring with the lasertracker a first distance to the first nest center; placing theretroreflector target in the second nest; sending the beam of lightthrough the air gap to the retroreflector target; measuring with thelaser tracker a second distance to the second nest center; anddetermining a first artifact length based at least in part on the firstdistance and the second distance.
 11. The method of claim 10, furthercomprising: setting a reference length for the length artifact equal tothe first artifact length; providing a 3D instrument capable ofmeasuring 3D coordinates of the first nest center and the second nestcenter; measuring with the 3D instrument third 3D coordinates of thefirst nest center; measuring with the 3D instrument fourth 3Dcoordinates of the second nest center; determining a test length basedat least in part on the third 3D coordinates and the fourth 3Dcoordinates; and determining a 3D instrument error based at least inpart on the test length and the reference length.
 12. The method ofclaim 11, wherein: the method further includes providing a coefficientof thermal expansion (CTE) of the length artifact; the step of providingthe length artifact further includes providing a temperature sensorattached to the length artifact, the temperature sensor configured totransmit an electrical signal indicative of a test temperature of thelength artifact; the method further includes receiving the electricalsignal and determining the test temperature based at least in part onthe electrical signal; and in the step of determining the first artifactlength, the first artifact length further depends on the testtemperature.
 13. The method of claim 10, wherein: in the step ofproviding a laser tracker, the distance meter is an interferometer; andin the step of placing the retroreflector target in the second nest, thelaser tracker tracks the retroreflector target as it is moved from thefirst nest.